Oil is usually driven out of oil-bearing formations into one or more wells by the natural pressure prevailing in said formation. This oil recovery is called the primary recovery. After these natural forces have become too small for further economical recovery, a considerable quantity of oil is still present in the formation. By injecting a fluid, frequently water or brine, into one or more injection wells some of this remaining oil can be forced out of the formation into one or more production wells. This further oil recovery is called the secondary recovery.
However, there is still a major portion of oil left in the formation after secondary recovery. Many efforts have been made to economically recover this remaining oil. One such method is known as surfactant flooding. In this method liquid solutions or dispersions containing one or more surfactants which have a very low interfacial tension against the crude oil are introduced into the formation. However a large quantity of residual oil often remains in the small pores of the formation even after surfactant flooding.